Several new ribonucleoside 3',5'-cyclic phosphates have been synthesized. These include cIMP 8-isoselenouronium hydrobromide, 8-Se-cIMP and 8-Se-cIMP. The anti-tumor effect and human platelet aggregation effect were investigated. It was found 8-Se-isouronium cIMP-HBr inhibits significant inhibiting activity against L5178Y, L1210 and ALL. The 3',4'-cyclic nucleotides, cGMP, cIMP and their 8-bromo-substituted analogs (100 microns) cause only slight inhibition (less than 10%) of ADP-induced platelet aggregation, whereas moderate inhibition (30-40%) is seen with the 8-seleno-substituted analogs. Most effective are 8-selenoisouronium-cIMP and 8-selenoisouronium-cGMP which cause 95-100% and 78-80% inhibition respectively on prolonged incubation (greater than 15 min). It is planned during the next year: the more new cyclic nucleotides will be synthesized. It is also anticipated that all the new cyclic nucleotides will be evaluated in several biological systems, cell cultures, enzyme preparations and several transplantable tumor systems.